A fuel cell is a system for producing electric power. In the fuel cell, chemical reaction energy between oxygen and hydrogen contained in hydrocarbon-group materials (e.g., methanol, natural gas) is directly converted into electric energy. Such a fuel cell is characterized by the production of electric energy and thermal energy as a by-product of an electrochemical reaction occurring without combustion.
Depending on the type of electrolyte used in a fuel cell, the fuel cell may be classified into one of many different types of fuel cells, for example, phosphate fuel cells, molten carbonate fuel cells, solid oxide fuel cells, and polymer electrolyte or alkali fuel cells. Although each of these different types of fuel cells operate using the same principles, they differ in the type of fuel, catalyst, and electrolyte used, as well as in drive temperature.
A polymer electrolyte membrane fuel cell (PEMFC) has been developed recently. Compared to other fuel cells, the PEMFC has excellent output characteristics, a low operating temperature, and fast starting and response characteristics. The PEMFC may be used for vehicles, in the home and in buildings, and for the power source in electronic devices. The PEMFC, therefore, has a wide range of applications.
The basic components of the PEMFC are a stack, a fuel tank, and a fuel pump. The stack forms the main body of the fuel cell. The fuel pump supplies to the stack fuel reserved in the fuel tank. A reformer may also be used to reform the fuel to create relatively pure hydrogen gas and to supply the hydrogen gas to the stack.
In the PEMFC, the fuel pump operates to send the fuel from the fuel tank to the reformer. The fuel is reformed in the reformer to generate hydrogen gas, and the hydrogen gas is chemically reacted with oxygen in the stack to generate electric energy.
Fuel cells using direct methanol fuel cells (herein referred to as “DMFC”) supply liquid methanol fuel containing hydrogen directly to the stack and therefore would not include a reformer. This lack of a reformer is the difference between the PEMFC and DMFC.
FIG. 7 is a partial cross section of a stack used in a fuel cell system according to the prior art where MEAs are assembled with separators. The stack may be composed of a structure of stacked unit cells. The stacked unit cells may contain a few unit cells, or ten or more unit cells having MEAs 51 and separators 53a, 53b. 
The MEAs 51 have an electrolyte membrane, and an anode electrode and a cathode electrode mounted on opposite surfaces thereof. The separators 53a, 53b have respective passages 55, 57 through which the hydrogen gas and/or air needed for the oxidation/reduction reaction of the MEAs 51 is supplied to the anode electrode and the cathode electrode. That is, the hydrogen gas is supplied to the anode electrode and the air is supplied to the cathode electrode through passages 55, 57, respectively, of the separators 53a, 53b. In this process, the hydrogen gas oxidizes at the anode electrode and the oxygen reduces at the cathode electrode. The flow of electrons generated during this operation creates a current. In addition, water and heat are generated by the electrochemical reactions.
In more detail, each separator 53a, 53b includes plurality of ribs 59 closely faced against the adjacent surfaces of MEAs which define the passages 55, 57 for supplying the hydrogen gas and air needed. Substantially, the passages are interposed between each of the ribs 59.
Typically, where separators are positioned on both sides of MEAs 51, the passages 55, 57 for supplying each of the hydrogen gas and air needed are orthogonal to each other. Thus, in the cross section illustrated in FIG. 7, a single passage 55 for supplying the hydrogen gas is illustrated while a plurality of passages 57 for supplying air are illustrated.
In the fuel cell system described above, the structure of a stack should enhance the diffusing performance in the stack while maintaining the pressure of the fuel during diffusion in order to enhance the efficiency of the fuel cell. Here, one important condition for designing the structure of a stack is the size and the number of the passages 55, 57. That is, in the separators 53a, 53b, the size and the number of passages 55, 77 plays an important role in diffusing hydrogen gas and air to diffusing layers thereof from the active area of the MEA 51, and also for handling the contact resistance of current generated in the MEA 51.